1. Technical Field
The invention relates to a communication apparatus capable of conducting communications of a multicarrier communication system such as an OFDM (Orthogonal Frequency Division Multiplexing) system, for example.
2. Background Art
The multicarrier communication system for transmitting information using a plurality of subcarriers, such as the OFDM system has the large advantage that it enables high-quality communications even in a harsh channel, and can be used for communications using various communication media, such as wireless communications and power line communications (PLC). The OFDM system is a system for using orthogonal transformation such as FFT (Fast Fourier Transform) or DWT (Discrete Wavelet Transform), making a plurality of carriers orthogonal to each other on a frequency domain, placing them at equal spacings, and multiplexing signals in parallel for transmission. In the OFDM system, the frequency interval of multicarrier can be narrowed, a plurality of carriers can be densely arranged without interfering with each other while some carriers overlap each other, and it is made possible to realize broad-band transmission efficiently using a narrow frequency range.
FIG. 1 shows a packet format example of a general OFDM signal. A packet of an OFDM signal transmitted in OFDM communications has a preamble 101, frame control (FC) 102, and a payload 103 from the head.
The preamble 101 is a separation bit string (synchronization code word) added to the beginning of data to serially transmit digital data and is formed of known data. For example, as the preamble 101, a preamble provided by providing each subcarrier with a plurality of “+1” consecutively or the like is used. A receiving communication apparatus uses the preamble formed of known data to perform control processing of carrier detection, synchronization timing extraction, equalization coefficient extraction, etc., for detecting the packet. The frame control 102 contains control information of tone map index, packet length, source address, destination address, index, etc., required for transmission and reception. Usually, fixed and strong error correction system, modulation system, time frequency diversity, etc. are applied to the frame control 102. The tone map is collection of information of the number of bits of each subcarrier (modulation system), error correction system, etc., and is previously determined and set between the transmitting communication apparatus and the receiving communication apparatus. The payload 103 is a portion of the data main body to be transmitted and the tone map data and the packet length data in accordance with the control information are stored in the packet and are transmitted.
When the packet of the OFDM signal as described above is transmitted from the transmitting communication apparatus, if new information 104 comes from an upper layer of the network after the packet length is determined and is specified in the control information of the control frame 102, the packet length is already determined and thus the new information cannot be added to the payload 103. In this case, the new information 104 is postponed and is transmitted in the payload 103 of the next frame. Since the preamble 101 and the frame control 102 are redundant information, if a new packet is generated to transmit the new information 104, overhead increases and the transmission efficiency is reduced. In communications requiring a real-time property, postponing the new information 104 to the next frame may lead to a signal delay (see JP-A-2004-166217, for instance).